Avocado root rot is the limiting factor for the growth of avocados throughout the world. Avocado root rot is caused by the fungus Phytophthora cinnamomi, which attacks and kills the feeder roots of avocado trees. The resultant lack of roots causes the tree to eventually die from water stress. There are a number of varieties of rootstocks that have some tolerance to the disease. These varieties included ‘Duke 7’ (unpatented), the most commonly planted tolerant rootstock in the world; and ‘Thomas’ (U.S. Plant Pat. No. 6,628), another root rot tolerant rootstock. However, even with these rootstocks, growers must still use a variety of methods, including mounding, mulching and the applications of chemical fungicides, to keep the tress from dying in many soils. More resistant rootstocks are necessary to eliminate avocado root rot as a major disease threat.
Screening and Greenhouse Evaluation of Rootstocks
‘Steddom’ was identified and characterized using the following screening protocol. As it is difficult to breed avocados because only one in approximately one thousand flowers actually set fruit, plant breeding blocks of avocados were isolated to prevent out-crossing with susceptible rootstocks. The breeding blocks were made up of various combinations of selected rootstocks including, ‘Thomas’ (U.S. Plant Pat. No. 6,628), ‘Barr Duke’ (U.S. Plant Pat. No. 6,627), ‘G6’, ‘Duke 7’, ‘Duke 9’, ‘UC 2001’, ‘UC 2011’, ‘Toro Canyon’ (U.S. Plant Pat. No. 5,642), ‘Spencer’, ‘CR1-71’, ‘G 810’, ‘G 875’, ‘G 755A’, ‘VC 256’, and ‘Steyemarkii’. In order to synchronize blooming, attempts were made to girdle late-blooming varieties and spray early-blooming varieties with the pesticide Unicona-zole-P.
Initial screening was carried out by germinating seeds, which were harvested from the breeding blocks, in flats of vermiculite in the greenhouse. Phytophthora cinnamomi-infested millet was placed in rows along with the young roots of the test seedlings. After 8-10 weeks roots were evaluated and those with a high percentage of surviving roots were transplanted to soil mix incorporated with P. cinnamomi-infested millet. Rootstocks that survived this test were planted and grown in P. cinnamomi-infested soils. Survivors were examined more carefully for various types of resistance using asexual propagated material.                a. Root survival—Rootstocks were grown in typical California avocado soils, inoculated with P. cinnamomi and evaluated for growth, root length and percent healthy roots.        b. Root regeneration—Rootstocks were grown in soil inoculated with P. cinnamomi, treated with Aliette to halt Phytophthora root rot and evaluated for root regeneration.        c. Attraction to P. cinnamomi—Roots of the rootstocks were placed in water baths with motile zoospores of P. cinnamomi. The numbers of spores attracted to the roots were evaluated.        
Rootstocks that performed well in the screening and greenhouse evaluations were further tested under field conditions.
Selection of ‘Steddom’
‘Steddom’ was developed at Riverside, Calif. The maternal parent is ‘Toro Canyon’ (U.S. Plant Pat. No. 5,642) avocado variety. The pollen parent is unknown. Specifically, the ‘Steddom’ rootstock variety was selected in 1994 from an agricultural operations land located Riverside, Calif. The fruit were collected from the avocado breeding blocks, the seed removed, and planted in vermiculite. The seeds were grown in a greenhouse. The plants were inoculated with the fungus Phytophthora cinnamomi. After showing tolerance to the disease, ‘Steddom’ was selected as a single plant for further testing. Budwood was collected from the plants and grafted to the stumps of adult avocado trees that had been cut down at Irvine Calif. The new varieties grew into trees which provided budwood for further testing. At least two ‘mother’ trees of the variety are growing in Irvine Calif., along with the germplasm. During screening and evaluation, ‘Steddom’, which was selected and originally designated ‘PP24’, distinguished itself from other varieties, including the maternal parent ‘Toro Canyon,’ by having a high tolerance against Phytophthora root rot. The properties of ‘Steddom’ were found to be true to type and transmissible by asexual reproduction.